This patch attempts to do a few things. The highlights are: 1) enable
SPARSE_IRQ unconditionally, 2) kills off !SPARSE_IRQ code 3) allocates
ivector_table at boot time and 4) default to cookie only VIRQ mechanism
for supported firmware. The first firmware with cookie only support for
me appears on T5. You can optionally force the HV firmware to not cookie
only mode which is the sysino support.

The sysino is a deprecated HV mechanism according to the most recent
SPARC Virtual Machine Specification. HV_GRP_INTR is what controls the
cookie/sysino firmware versioning.

The history of this interface is:

1) Major version 1.0 only supported sysino based interrupt interfaces.

2) Major version 2.0 added cookie based VIRQs, however due to the fact
   that OSs were using the VIRQs without negoatiating major version
   2.0 (Linux and Solaris are both guilty), the VIRQs calls were
   allowed even with major version 1.0

   To complicate things even further, the VIRQ interfaces were only
   actually hooked up in the hypervisor for LDC interrupt sources.
   VIRQ calls on other device types would result in HV_EINVAL errors.

   So effectively, major version 2.0 is unusable.

3) Major version 3.0 was created to signal use of VIRQs and the fact
   that the hypervisor has these calls hooked up for all interrupt
   sources, not just those for LDC devices.

A new boot option is provided should cookie only HV support have issues.
hvirq - this is the version for HV_GRP_INTR. This is related to HV API
versioning.  The code attempts major=3 first by default. The option can
be used to override this default.

I've tested with SPARSE_IRQ on T5-8, M7-4 and T4-X and Jalap?no.
Signed-off-by: Bob Picco <bob.picco@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>